Based on the H2O phase diagram, which shows pressure versus temperature, what is a unique characteristic of water (H2O)?

Question

BlackTom AI · Accepted Answer

A phase diagram maps pressure vs temperature showing regions where solid, liquid, and gas are stable; the three lines separating those regions meet at the triple point. The triple point is the single set of pressure and temperature at which solid, liquid, and gas can coexist in equilibrium.

Step-by-step reasoning:
1. Identify the defining feature on a P–T phase diagram: intersection of the three phase boundaries is the triple point. For pure H2O that point occurs at T = 0.01°C (273.16 K) and P = 611.657 Pa.
2. Convert the triple-point pressure to familiar units (showing intermediate steps):
   - In pascals it is 611.657 Pa (given by thermodynamic tables).
   - In atmospheres: 611.657 Pa ÷ 101325 Pa/atm = 0.0060366 atm.
   - In millibars (hPa): 611.657 Pa ≈ 6.11657 hPa.
   These calculations illustrate that the triple point is a single, well-defined pressure and temperature.
3. Why answer d is correct: the phase diagram explicitly shows that at one unique P and T the three phase boundaries intersect — that is the triple point where ice, liquid water, and vapor coexist in equilibrium.

Why the other options are incorrect:
- a. False: increasing pressure on liquid water generally raises its boiling point (Clausius–Clapeyron relation). The melting point of water decreases with pressure (negative slope of the solid–liquid line), but that is about melting, not boiling.
- b. False: saturation vapor pressure over ice is lower than over supercooled liquid at the same temperature (so the statement’s direction is wrong).
- c. False: water can be liquid above 0°C, and can also be supercooled (liquid below 0°C); it is not restricted to solid below 0°C.
- e. False: the boiling point depends on atmospheric pressure; only at 1 atm is it approximately 100°C.

Therefore the correct answer is d. This matches the provided answer: d. It has a single "triple point" where all three states (solid, liquid, gas) can simultaneously coexist in equilibrium.

Based on the H2O phase diagram, which shows pressure versus temperature, what is a unique characteristic of water (H2O)?单项选择题

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